Was there a liner somewhere in that mix, or just deck mud on the dirt? Some people consider the tile as a waterproof layer, but it is not. All current codes require a sloped, waterproof layer underneath the tile that is then tied into the drain. For stability, I think I'd want to pour a slab, tied into the existing slab with rebar drilled into the existing slab and epoxied in place. If you were considering a barrier free shower, this would be an ideal time to do it, since it would then be easier to recess the pan and get the slope you need for one.